Implement CSS-only input toggling, refactor related forms

UX/Translation changes:

- new teams: remove redundant tooltips that don't add meaningful information
  - move general information to table fieldset
- new teams: rename "general" to "custom" access for clarity
- new teams: show labels beside options on mobile

Accessibility:

- semantic form elements allow easier navigation (fieldset, mostly)
- improve better labelling of new teams table
- fix accessibility scan issues
- TODO: the parts that "disable" form elements were not yet touched and
  are not really accessible to screenreaders

Technical:

- replace two JavaScript solutions with one CSS standard
- implement a simpler grid (.simple-grid)
- simplify markup
- remove some webhook settings specific CSS

Testing:

- check more form content for accessibility issues
- but exclude tooltips from the scan :(
- reuse existing form tests from previous PR

web_src/css/repo.css

@@ -1805,16 +1805,6 @@ td .commit-summary {
   font-style: italic;
 }
 
-.repository.settings.webhook .events .column {
-  padding-bottom: 0;
-}
-
-.repository.settings.webhook .events .help {
-  font-size: 13px;
-  margin-left: 26px;
-  padding-top: 0;
-}
-
 .repository .ui.attached.isSigned.isWarning {
   border-left: 1px solid var(--color-error-border);
   border-right: 1px solid var(--color-error-border);